Plywood which is used extensively in the building trade, is produced from stacked and laminated sheets of veneer. For example, a 4'.times.8' five ply sheet of plywood 5/8" thick may be produced from five separate 4'.times.8' veneer sheets, each being 1/8" thick, stacked one on top of the other and glued together. Certain of the inner ply layers may be produced from butted partial veneer sheets, e.g. two veneer half sheets of 2'.times.8' or two veneer sheets of 4'.times.4', etc., which in any event make up a full inner ply of 4'.times.8'. For the sake of accuracy, when making up the sheets of plywood, the dimensions are 54".times.102" (or 27".times.102" for half sheets) which are subsequently trimmed back to 4'.times.8' sheets.
The above dimensions are the most common for wood veneer used to produce plywood sheets. However, they are but examples of the product that is produced in a veneer peeling process to which the present invention is applicable. As already explained, when reference is made to 4'.times.8' sheets, the reader should understand that a full sheet of veneer or plywood is intended and in the pre-trimmed stage, those dimensions will actually be 54" .times.102". Similarly, as related to the uncommon production of plywood having totally different dimensions (e.g. 5'.times.9'), those different dimensions can be substituted for the 4'.times.8' dimensions in the described examples.
The process of producing the individual veneer sheets for the examples given, typically involves the peeling of a continuous sheeting of veneer from an 8' block, thus producing an 8' wide continuous sheeting. Other block lengths of 4', 6' and 10' are, however, also peeled for veneer sheeting and this invention is not limited to any particular block length or to a particular width or even length of veneer sheet to be produced from the different block lengths. The continuous sheeting of veneer is directed through a clipper that cuts the veneer to a desired sheet length, e.g. of 4', thus producing the common 4'.times.8' full sheets. Note, as concerns the sheet handling process, that the length dimension of the sheet is considered herein to be the direction of sheet movement, and the width dimension is considered to be the side edge to side edge dimension, thereby producing a sheet that is 8' wide and 4' long.
Whereas the peeling and handling of veneer sheeting (continuous) and veneer sheets (individual) has been largely automated, one area that has continued to plague veneer producing mills is the occurrence of offsize defects in the sheeting. An offsize defect is a change in thickness. Such a defect occurs when in effect, the peeling knife digs in too deep or lifts away from the block being peeled. There are many situations that cause such erratic behavior during the peeling operation. Whereas considerable attention is paid to avoid these situations, the defects nevertheless do occur and when they do, unless detected and removed, they will result in the production of flawed plywood. The occurrence of a defect in the sheeting must be accommodated in the downstream handling of the veneer sheets.
This problem of offsize defect is to be distinguished from defects such as cracks, knot holes and the like where there is some portion missing from the sheeting of veneer. Such defects are typically detected by a light bar projected across the width of the veneer sheeting. A light detector on the opposite side of the sheeting will "see" the light projected through any opening in the sheet and will activate the clipper to cut out the flaw. Of course, such a detector (e.g. an occlusion type scanner) cannot detect offsize defects. It is the objective of the present invention to detect offsize defects as differentiated from breaks or openings through the sheeting.
Returning to the detection of offsize defects, the automatic production of plywood depends on a consistent thickness of the veneer sheets. If sheets having offsize defects are introduced into the plywood laminating process, the plywood will end up with depressions or bulges at the cross section of the offsize defect, either of which is unacceptable.
The presence of even a small percentage of defective plywood produced by a mill can be enormously expensive and is to be avoided. Accordingly, it is highly desirable that defective veneer sheets be pulled out of the plywood producing process before it reaches the lamination stage of production. A single defect in one veneer sheet that is laminated into a plywood sheet will reduce the entire five sheets of veneer to near worthless scrap. The stacking, drying and peeling operations are, of course, wasted.
Detection and removal of the defective veneer sheet will not only save the other four veneer sheets and avoid the wasted gluing step, but the defective sheet may even be partially saved by trimming out the defective portion. It is therefore an object of the present invention to automatically detect and mark the location of the offsize defects and in response thereto, automatically divert sheets containing such defects for subsequent processing.